Hydrostatic plain bearings

ABSTRACT

An arrangement of plain hydrostatic bearings for rotatably mounting a spindle in which the spindle is provided with a pair of tapered bearing surfaces symmetric to a plane perpendicular to the spindle&#39;&#39;s axis. Front and rear bearing boxes, each formed by several segments having lubricating oil pockets, are formed about the tapered bearing surfaces with the front box being rotatably and axially fixed and the rear box being rotatably fixed but axially movable to compensate for heat expansion.

[151 3,659,911 1 May 2,1972

United States Patent Kessler et al.

[54] HYDROSTATIC PLAIN BEARINGS FOREIGN PATENTS OR APPLlCATlONS 72Inventors: FranzKessler; Karl Brand, bothofEbern, 332,253 10/1958Germany Primary ExaminerMartin P. Schwadron Assistant Examiner-FrankSusko Attorney-Stevens, Davis, Miller & Mosher ABSTRACT An arrangementof plain hydrostatic bearin [22] Filed:

gs for rotatabl y mounting a spindle in which the spindle is providedwith a pair of tapered bearing surfaces symmetric to a planeperpendicuont and rear bearing boxes, each having lubricating oilpockets, are

[30] Foreign Application Priority Data lar to the spindle's axis. Frformed by formed ab July 7, 1969 Germany......................P i9 34361.4 several segments out the tapered b earing surfaces with the frontbox 308/122 being rotatably and axially fixed and the rear box being --F16 rotatably fixed but axially movable to compensate for heat ex-.308/5, 9 pansion.

[51] Int. [58] Field ofSearch 3 Claims, 4 Drawing Figures ReferencesCited UNITED STATES PATENTS 2,389,687 11/1945Rickenmann..............................308/5 Patented May 2, 19723,659,911

2 Sheets-Sheet l INVENTORS FRANZ KESSLER KARL BRAND TTORNEYS PatentedMay 2, 1972 3,659,911

2 Sheets-Sheet 2 HYDROSTATIC PLAIN BEARINGS The present inventionrelates to a means for mounting a spindle by hydrostatic plain bearings,intended particularly for the work spindles of machine tools, whichconsists of tapered supporting slide surfaces formed on the spindle bywhich it is radially and axially supported in corresponding conicalnonrotating bearing bushings provided with lubricating oil pockets.

The use of hydrostatic plain bearings in supporting work spindles ofcutting machine tools and the like is becoming more and more popular asa result of the high overall stiffness of the supported system which canbe obtained, the high range of sliding speed and the high degree oftrueness of running of the supported spindle.

There are already known hydrostatic plain bearings for supportingspindles which consist of full-shell conical bearing bushings screwedonto the spindle and resting in corresponding conical bearing boxeswhich are fixed against rotation. There are lubricating oil pockets inthese boxes into which lubricating oil is forced to then emerge from thelubricating oil pockets through the gap between the bearing slidesurfaces. Both the front bearing, i.e., the one which lies at the end ofthe spindle upon which the forces occurring during the cutting work act,as well as the opposite or rear bearing, are developed as fixed bearingsin the known spindle mountings. There is a spindle bushing in thebearing box located opposite each of the bearings with the bushingsbeing tapered conically towards each other, the bearing boxcorrespondingly narrowing or broadening in a tapered shape towards itscenter.

. If the distance between bearings increases, due to thermal expansionof the spindle caused by operation, then a change is produced in thewidth of the lubricating gaps in the known spindle mounting. Thelubricating slot at the inner conical bearing slide surfaces, seen inaxial direction, becomes so narrow that there is the danger of metallicpockets. If contact, while the outer conical bearing lubricating slotbecomes too wide to be effective. This change in the width of thelubricating slot results in a change of the pressure in the lubricatingoil pockets, which makes it difficult, if not impossible, to maintainthe required oil pressure in the pockets for carrying the spindle.Furthermore, in the known spindle mounting the unipartite bearing boxesare replaced only by cumbersome removal of the bearing bushings whichare screwed onto the spindle. If the front end of the spindle isprovided with a flange cut thereon, then the spindle part locatedbetween the bearings cannot be increased in diameter, since in such casethe unipartite bearing bushings and the bearing box could not beassembled.

These disadvantages are eliminated in a hydrostatic spindle mountinghaving tapered bearing slide surfaces in that the spindle is formed withtwo tapered bearing slide surfaces symmetrical to a plane perpendicularto the axis of the spindle and that the bearing boxes are composed of atleast three segments of the same conicity, each provided with onelubricating oil pocket. The segments are clamped firmly in position inthe housing for the front spindle bearing. For a rear spindle bearing,the segments are brought by axial displacement over a clamping device,having an initial stress, supported on the housing onto thecorresponding tapered bearing slide surface of the spindle.

In accordance with the present invention, the spindle to be supportedhas a tapered bearing slide surface on both its front and rear ends withthese surfaces resting on corresponding bearing boxes. In thisconnection the bearing box, composed of several segments, of the frontspindle bearing is finnly clamped and axially fixed in the housing inthe same manner as in the case of a fixed bearing, while the bearing boxof the rear spindle bearing, which box also consists of a plurality ofsegments, is displaceable axially. In accordance with the presentinvention there is furthermore provided a clamping device having aninitial stress which rests on the housing of the spindle mounting andwhich urges the bearing box segments of the rear spindle bearing axiallyagainst the corresponding conical bearing slide surface of the spindle.A lubricating gap is formed between the bearing box elements providedwith pockets and supplied with oil under pressure and the bearing slidesurfaces of the spindle to prevent metallic contact between the partswhich slide on each other.

The spindle mounting of the present invention has the advantage that therequired widths of the lubricating gap are maintained constant by anautomatic axial readjustment of the bearing box segments of the rearspindle bearing when ther mal expansion of the spindle resulting fromoperation occurs. The width of the lubricating gap of the rear spindlebearing is not increased despite any increase in length of the spindle.This is important since the amount of oil under pressure required tomaintain the lubricating oil pocket pressure would increase by the thirdpower as the width of the slot increases in the known hydrostaticbearings with tapered slide surfaces. In contradistinction to this, thespindle mounting of the present invention has constant lubricating slotwidth so that the feed power for the pressure oil can remain constant,and this leads to a simplification of the unit for the supplying of oilunder pressure.

In accordance with the present invention, the automatic axialreadjustment of the bearing box segments in order to maintain a constantlubricating slot width is produced by an axially acting clamping devicehaving an initial stress which produces a predetermined axial force theamount of which depends on the required hydrostatic pressure in thelubricating oil pockets. If this clamping device consists of amechanical spring, then the required spring hardness depends on thepressure in the lubricating oil pockets. By a change in the initialstress of the clamping device, the lubricating slot width between thesurfaces of the spindle mounting can be optimally adapted to thespecific operating conditions.

In the case of axially displaceable bearings in accordance with thepresent invention, the following relationship can be set up for theoptimum adaptation of the lubricating slot width after determination ofthe bearing stiffness which is dependent in particular also on thepocket pressure and after the establishing of the speed of sliding.

1...: a-Man P P -b Herein:

1 viscosity of the oil v speed of sliding l flow-out length b flow-outwidth P,,= pump pressure P, pocket pressure F sum of the rib surfaces e,pump efficiency It will be noted from the above relationship that theoptimum slot width for given bearing dimensions, as well as a givenviscosity of the oil and given pump and pocket pressures, depends verysubstantially on the speed of sliding.

The conical bearing boxes of the inventive spindle mounting are not madeof one piece but are composed of at least three segments. The spindlepart between the two bearings is thus not limited in diameter as in thecase of the known arrangements. These bearing box segments can be usedeven in the case of spindles which are of considerably increaseddiameter between the bearing places, as well as in the case of spindlesthe front end of which have a flange cut thereon. An increase in thediameter of the spindle leads to an increase in the moment of inertia ofthe spindle and thus in the flexural stiffness of the spindle. A spindleshould be sufiiciently rigid that it is only insignificantly defonned bythe forces which occur. The fact that a thickening of the spindlebetween the supporting places increases the stiffness can be noted fromthe wellknown Schenk formula:

in which R, is the degree of rigidity, D the outside diameter of thespindle between the bearings, d the diameter of the spindle bore, and Ithe distance between bearings. In the spindle mounting of the invention,in which the spindle is not limited in its moment of inertia by themounting of the bearing, there is no danger therefore, upon the actionof high radial loads, of metallic contact between a bent spindle and theedges of the bearing box.

The bearing slide surfaces of the spindle preferably taper towards eachother, with each surface having the same taper angle. In accordance withanother feature of the inventive mounting, a ring which is axiallydisplaceable in the bearing housing can be arranged between the axiallydisplaceable bearing box segments of the rear spindle bearing and theclamping device.

The clamping device for the bearing box segments of the rear spindlebearing can consist of a plate spring, or a plurality of spiral springswhich are distributed over the periphery of a grooved intermediate ring,or finally a hydraulically actuated ring-piston unit. An adjustingsleeve provided with external thread which rests on a fixed threadedring of the housing and presses against the different spring elementscan be provided for adjusting and changing of the initial tension of theclamping device.

The means for accomplishing the foregoing objects and other advantages,which will be apparent to those skilled in the art, are set forth in thefollowing specification and claims and are illustrated in theaccompanying drawings dealing with a basic embodiment of the presentinvention. Reference is made now to the drawings in which:

FIG. I is an axial section through a spindle mounting in accordance withthe invention;

FIG. 2 is a section along the line 22 of FIG. 1,

FIG. 3 is a partial axial section through a spindle mounting having aclamping device different from that of FIG. 1; and

FIG. 4 is a partial axial section through a spindle mounting having astill different clamping device.

In accordance with FIG. 1, a spindle 2 which is to be supported isradially and axially guided in a housing 1 by means of two conicalbearing slide surfaces 3 in hydrostatic plain bearings. Between thebearing slide surfaces 3, the central portion 4 of the spindle has thelargest possible diameter in order to increase the stiffness andrigidity of the spindle. The conical bearing slide surfaces 3 of thespindle 2 rest on corresponding tapered bearing boxes, each of which iscomposed, in the embodiment shown by way of example, of four segments 5(see FIG. 2). Each of these segments 5 is provided with an oil pocket 6supplied with oil under pressure through feed bore holes 7 from ahydraulic source (not shown). The pressure in the lubricating oilpockets is 40 to 120 atm.ga., depending on the size of the hearing. Thebearing box segments of the front spindle bearing are clamped by a lid 8in the housing 1 and form the fixed bearing. The bearing box segments ofthe rear spindle bearing are seated in a ring 9 which is axiallydisplaceable in the housing 1.

FIG. 2 shows pins 10 which are provided to secure the bearing boxsegments 5 against rotation and fix the distance between the segments.

The clamping device of the invention for the automatic axialreadjustment of the bearing box segments of the rear spindle bearingconsists, in accordance with FIG. I, of a plate spring 11, and, inaccordance with FIG. 3, of coil springs 13 which are distributed overthe periphery of a grooved intermediate ring 12. Each of these springsrests against a front side of the axialy displaceable ring 9. Foradjusting and changing the initial tension of the spring elements thereis an adjusting sleeve 14 provided with external threads and which restsin a fixed threaded ring 15 of the housing I and presses against thedifferent spring elements. The housing 1 has a lid 17 on a top portiongiving access to sleeve 14 for the adjustment thereof. The oil ofreduced pressure emerging from the lubricating oil pockets 6 through thelubricating slots is fed back via bore holes I6 to the hydraulicpressure-oil suppl unit.

In FIG. 4, the clamping is effected wit the aid of means other than theplate spring 11 of FIG. I or the coil spring 13 of FIG. 3, viz., withthe aid of the ring part 18 actuated by the piston 19 moving to the leftunder the pressure of oil from a conventional hydraulic source (notshown).

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiment is therefore to be con sidered in all respects asillustrative and not restrictive. the scope of the invention beingindicated by the appended claims rather than the foregoing description,and all changes which come within the meaning and range of equivalencyof the claims are therefore to be embraced therein.

We claim:

1. A spindle support with hydrostatic plain bearings, particularly forwork spindles of machine tools, consisting essentially of front and rearconical bearing slide surfaces of a rotating spindle to be supported bywhich it is radially and axially supported on correspondingly front andrear conical bearing shells which are fixed against rotation andprovided with lubricating oil pockets, and wherein the spindle has twoconical bearing slide surfaces symmetrical to a plane perpendicular tothe spindle axis, said surfaces tapering towards each other, and each ofthe bearing shells consisting of at least three segments of the sametaper, each of the segments being provided with a lubricating oilpocket, said segments of said front shell being clamped fast at thefront spindle bearing within a housing and the rear bearing shell beingpressed in axially displaceable manner by a prestressed clamping devicesupported on the housing against the rear conical bearing slide surfaceof the spindle.

2. A spindle support according to claim 1, wherein the clamping deviceconsists essentially of a plurality of hydraulically actuatedpiston/cylinder units distributed over the circumference of the spindle.

3. A spindle support according to claim 1, wherein for the adjustmentand change of the initial tension of the clamping device there isprovided an externally threaded adjusting sleeve which rests in a fixedthreaded ring of the housing and presses against spring elementscomprising the prestressed clamping device.

w a x a I:

1. A spindle support with hydrostatic plain bearings, particularly forwork spindles of machine tools, consisting essentially of front and rearconical bearing slide surfaces of a rotating spindle to be supported bywhich it is radially and axially supported on correspondingly front andrear conical bearing shells which are fixed against rotation andprovided with lubricating oil pockets, and wherein the spindle has twoconical bearing slide surfaces symmetrical to a plane perpendicular tothe spindle axis, said surfaces tapering towards each other, and each ofthe bearing shells consisting of at least three segments of the sametaper, each of the segments being provided with a lubricating oilpocket, said segments of said front shell being clamped fast at thefront spindle bearing within a housing and the rear bearing shell beingpressed in axially displaceable manner by a prestressed clamping devicesupported on the housing against the rear conical bearing slide surfaceof the spindle.
 2. A spindle support according to claim 1, wherein theclamping device consists essentially of a plurality of hydraulicallyactuated piston/cylinder units distributed over the circumference of thespindle.
 3. A spindle support according to claim 1, wherein for theadjustment and change of the initial tension of the clamping devicethere is provided an externally threaded adjusting sleeve which rests ina fixed threaded ring of the housing and presses against spring elementscomprising the prestressed clamping device.